1. Field
This disclosure relates to rotatable polarizer devices for use in cylindrical waveguides.
2. Description of the Related Art
Satellite broadcasting and communications systems, such as Ku band very small aperture terminal (VSAT) communications systems, may use orthogonally polarized signals within the same frequency band for the uplink to and downlink from satellites.
A common form of antenna for transmitting and receiving signals from satellites consists of a parabolic dish reflector and a feed network where orthogonally polarized modes travel in a circular waveguide. Note that the term “circular” refers to the cross-sectional shape of the waveguide. An ortho-mode transducer may be used to launch or extract the orthogonal linearly polarized modes into or from the circular waveguides.
An ortho-mode transducer (OMT) is a three-port waveguide device having a common waveguide coupled to two branching waveguides. Within this description, the term “port” refers generally to an interface between devices or between a device and free space. A port may include an interfacial surface, an aperture in the interfacial surface to allow microwave radiation to enter or exit a device, and provisions to mount or attach an adjacent device.
The common waveguide of an OMT typically supports two orthogonal linearly polarized modes. Within this document, the terms “support” and “supporting” mean that a waveguide will allow propagation of a mode with little or no loss. In a feed system for a satellite antenna, the common waveguide may be a circular waveguide. The two orthogonal linearly polarized modes may be TE11 modes which have an electric field component orthogonal to the axis of the common waveguide. When the circular waveguide is partially filled with a dielectric material, the two orthogonal linearly polarized modes may be hybrid HE11 modes which have at least some electric field component along the propagation axis. Two precisely orthogonal TE11 or HE11 modes do not interact or cross-couple, and can therefore be used to communicate different information.
The common waveguide terminates at a common port aperture. The common port aperture is defined by the intersection of the common waveguide and an exterior surface of the OMT.
Each of the two branching waveguides of an OMT typically supports only a single linearly polarized TE10 mode. The mode supported by the first branching waveguide is orthogonal to the mode supported by the second branching waveguide. Within this document, the term “orthogonal” will be used to describe the polarization direction of modes, and “normal” will be used to describe geometrically perpendicular structures.
A satellite communications system may use a signal having a first polarization state for the uplink to the satellite and a signal having a second polarization state, orthogonal to the first polarization state, for the downlink from the satellite. Note that two circularly polarized signals are orthogonal if the e-field vectors rotate in the opposite directions. The polarization directions for the uplink and downlink signals may be determined by the antenna and feed network on the satellite. To ensure maximum coupling of the signals to and from the satellite, each terrestrial antenna may include provisions to adjust the polarization directions of the uplink and downlink signals to exactly match the polarization directions defined at the satellite. In present antennas, the polarization directions of the uplink and downlink signals may be adjusted by rotating the entire antenna or by rotating all or portions of the feed network including the OMT. In either case, the item being rotated is heavy and the cables connecting to the feed network must be repositioned.
U.S. Pat. No. 7,772,940 describes a feed network including an integrated OMT and polarization controller. A rotatable phase shifting element disposed in a common waveguide coupled to the common port of the OMT. The rotatable phase shifting element is configured to introduce a phase shift between two signals having orthogonal polarization states. The polarization of the uplink and downlink signals in the common waveguide may be precisely adjusted by rotating the rotatable phase shifting element.
When a satellite communications system uses orthogonal linearly polarized signals for the uplink to the satellite and the downlink from the satellite, the rotating phase shifting element may introduce a 180-degree phase shift to allow adjustment of the polarization direction of two orthogonal linearly polarized signals in the common waveguide of the feed network. When a satellite communications system uses orthogonal circularly polarized signals for the uplink to the satellite and the downlink from the satellite, the rotating phase shifting element may introduce a 90-degree phase shift to allow adjustment of the ellipticity of two orthogonal polarized signals in the common waveguide of the feed network.
FIG. 9 is a perspective, partially cross-sectional, view of a prior art rotatable phase shifting element 900 developed by the inventors of the present patent. The phase shifting element 900 is shown within a cylindrical waveguide 910 which terminates in flanges 912 and 914. The cylindrical waveguide 910 is shown in partial cross-section to allow the phase shifting element 900 to be seen. The phase shifting element 900 includes a dielectric card 920 which extends into a dielectric cylinder 930. The opposing ends of the dielectric card 920 and the dielectric cylinder 930 are coupled to stems 940, 942 which may be rotatable within bearings (not shown).
The dielectric card has a rectangular cross-section that spans or nearly spans the inside diameter of the cylindrical waveguide in a first direction and is much smaller in a second direction orthogonal to the first direction. The effect of the dielectric card is to slow propagation of a first electromagnetic wave polarized in the first direction with respect to a second electromagnetic wave polarized in the second direction. By selecting the proper length of the dielectric card, the phase of the first electromagnetic may be shifted with respect to the phase of the second electromagnetic wave by a desired amount such as 90 degrees or 180 degrees.
A structure such as the phase shifting element 900 may cause undesired resonances within the operating bandwidth of a feed network. In the phase shifting element 900, undesired resonances were suppressed by a series of irregularly spaced slots 925 in the dielectric card 920. The slots 925 were located by trial and error.
Elements in the drawings are assigned reference numbers which remain constant between the figures. An element not described in conjunction with a figure may be presumed to be the same as an element having the same reference number described in conjunction with a previous figure.